Liquid crystal devices incorporating a ferroelectric smectic liquid crystal material (FLCDs) are particularly suitable for use in displays and shutters in which their fast switching times and memory characteristics are of advantage. A conventional FLCD comprises a layer of ferroelectric smectic liquid crystal material between two parallel glass substrates, electrode structures being typically provided on the inside facing surfaces of the glass substrates in the form of row and column electrode tracks which cross one another to form a matrix array. As is well known, switching pulses are applied to the row and column electrode tracks in order to produce electric fields which switch the molecules within the material between two polar states having different molecular orientations. As a result of the different light transmitting properties of the two molecular orientations when the material is disposed between two polarisers having polarising axes which are arranged transversely to one another, a display element or pixel at the intersection of two tracks will appear dark or light depending on the state to which the molecules of the pixel have previously been switched by the electric field due to the voltage difference between the pulses applied to the relevant row electrode track on one side of the layer and the relevant column electrode track on the other side of the layer.
Various addressing schemes for controlling such a matrix array FLCD are known, and International Patent Application No. WO 92/02925 discloses a number of such addressing schemes. Typically, in large displays, the display is addressed on a line-by-line basis by applying data pulses in parallel to the column electrode tracks, each data pulse being either an ON pulse (i.e. a switching waveform) or an OFF pulse (i.e. a non-switching waveform), and by sequentially applying a strobe pulse to the row electrode tracks so as to switch selected pixels along each row from one state to the other under the effect of the electric field produced by the voltage difference between the data pulse and the strobe pulse applied to the relevant electrode tracks.
In certain addressing schemes a blanking pulse is applied sequentially to the row electrode tracks which is of such a voltage and duration as to set all the pixels along each row to one state, generally the dark state, irrespective of the data pulses applied to the column electrode tracks. Subsequent application of strobe and data pulses may then be used to switch selected pixels to the other state, generally the light state, whilst leaving the remaining pixels in the one state. Such blanking pulses may be applied on a line-by-line basis ahead of the strobe pulses, or the whole display may be blanked at a time, or a group of lines may be simultaneously blanked. A known blanking scheme uses a blanking pulse of voltage V and duration 2T and having the same product 2VT as, but opposite polarity to, the strobe pulse which has a voltage 2V and a duration T. This ensures that the blanking and strobe pulses are d.c. balanced which is an important requirement to avoid degradation of the display over time.
The advantage of using blanking pulses to switch a line, group of lines or whole display to the dark state before pixels are selectively switched to the light state on a line-by-line basis is that this allows the display to be switched with one strobe pulse frame scan, rather than with two strobe pulse frame scans, one for switching selected pixels to one state and the other for switching selected pixels to the other state, which would result in a lengthy total scan cycle. However a drawback of the blanking pulse scheme is that the pixels are switched fully to one state (usually the dark state) by the blanking pulse, so that those pixels which require to be subsequently switched to the other state during application of a subsequent strobe pulse have to be switched from the fully switched state to the other state, which takes longer than switching from a relaxed state into which the pixels would relax after a suitable delay following removal of the blanking pulse.
As is well known, after removal of the applied electric field, the orientation of the molecules of a pixel will gradually relax into a relaxed state which is close to the fully switched orientation of the molecules so that the pixel still appears dark, but which is part way along the rotational path followed by the director of the molecule during switching into the other state, so that subsequent switching into the other state occurs more rapidly from the relaxed state than it does from the fully switched state. However, if sufficient time is allowed between the blanking pulse and the subsequent strobe pulse to permit the molecules of the pixel to relax into the relaxed state, this provides a significant delay between scanning of a line by a blanking pulse and scanning of the same line by a subsequent strobe pulse, and, if the delay becomes too large, means that a blank line or group of lines becomes visible on the display. Furthermore this will result in a loss of brightness since those pixels which are intended to be in the light state will be in the dark state for a proportion of the addressing cycle. Similarly, if the blanking scheme is such as to switch the pixels to the light state, this will result in a loss of contrast since those pixels which are intended to be in the dark state will be in the light state for a proportion of the addressing cycle.
U.S. Pat. No. 5,283,564 discloses an addressing scheme in which the blanking pulse is followed by a strobe pulse, the strobe pulse having a control phase part followed by a compensation phase part of opposite polarity and synchronised with the corresponding data pulse. The compensation phase part of the strobe pulse serves to increase the temperature margin over which the display presents an acceptable image. However such a compensation phase part does not overcome the problem of the delay between the application of the blanking pulse and the subsequent strobe pulse.
Japanese Published Patent Application No. 2230118A discloses an addressing scheme in which the blanking pulse comprises a resetting part to switch all the pixels to the first state and a partial switching part which then causes all the pixels to incompletely switch to the second state such that the application of the subsequent strobe pulse synchronised with the data pulse enables the addressed pixels to be selectively switched into the first state according to a greyscale signal to display a half tone. Such an addressing scheme has the object of displaying grey levels in a uniform manner across the display so as to reduce the variation of such grey levels with temperature, etc. This reduction in temperature dependence is due to the fact that the grey level of each pixel is set by partially switching from the first state to the second state and by then returning to the first state, and these two switching operations will tend to compensate for the effects of temperature variation. Such an addressing scheme is not intended to promote rapid switching of the display.
It is an object of the invention to provide an improved method of addressing a matrix array liquid crystal device which allows rapid switching of the device whilst keeping the delay between the application of a blanking pulse and a subsequent strobe pulse relatively short so that it does not adversely affect operation of the device.